To meet the ever-increasing demand for high-speed Internet access and services, many network operators are deploying, or planning to deploy, passive optical networks. A passive optical network (PON) is a point-to-multipoint, fiber-to-the-premises, broadband network architecture in which unpowered (“passive”) optical splitters are used to enable a single optical fiber to serve multiple customer premise locations (this single fiber referred to at times as a “feeder fiber”). As shown in FIG. 1, a typical prior art PON 10 includes an optical line terminal (OLT) 12 at a service provider's central office (CO) and a multiplicity of optical network units or terminals (ONUs or ONTs, hereinafter simply referred to as “ONUs”) 14 located at or in the vicinity of end users (i.e. subscriber or customer premise locations). A single OLT 12 is optically coupled to a plurality of ONUs 14 via an optical distribution network (ODN) 16 comprising a transmission optical fiber 18 (the “feeder fiber”) that terminates at a remote node (RN) 20 located in relatively close proximity to ONUs 14. A 1:N passive optical splitter 22 is located within remote node 20 and is used to divide the arriving signal into a plurality of N sub-signals (in this example, N is 32) so as to complete the communication path to each separate ONU 14. The ODN is often referred to as the “outside plant”.
Even though it is apparent that the terminals (OLTs and ONUs) of the network include active components and/or circuits that require electrical power, a PON is said to be passive so long as the ODN portion of the network is passive (i.e., does not require electrical power), it is common in the industry to refer to the entire network as being passive.
The number of ONUs that communicate with a single OLT is determined by the split ratio (1:N) of power splitter 22. Each ONU 14 terminates the optical fiber transmission line and provides bidirectional communication with OLT 12. The ability to use only a single fiber between OLT 12 and splitter 22 is made possible by using wavelength division multiplexing (WDM) to maintain separation between the “downstream” and “upstream” signals (where “downstream” refers to those optical signals transmitted from OLT 12 to ONUs 14 and “upstream” refers to those optical signals transmitted from the various ONUs 14 to OLT 12). In a GPON system configured in accordance with industry standards (for example, ITU-T G.984.5, which defines the use of the “narrow wavelength” option for upstream signals), the downstream signals are transmitted at a wavelength in the range of 1480-1500 nm and upstream signals are transmitted at a wavelength in the range of 1300-1320 nm. Continuous-mode downstream signals (e.g., 1490 nm signals from OLT 12 to ONUs 14) are broadcast to each ONU sharing the single fiber 18; that is, a downstream signal is divided at splitter 22 into a multiplicity of N (in this example, N=32) sub-signals that are directed onto a multiplicity of N optical fiber paths 24 coupled to different ONUs 14 in a one-to-one relationship. It is to be understood that the sub-signals at the output of splitter 22 are essentially identical to the downstream signal as received at the input of remote node 20, but have lower power due to the inherent function of the splitter.
In contrast to the use of continuous-mode signaling for downstream transmissions, burst-mode transmission is used for the upstream signals created at the various ONUs 14 and directed to the single OLT 12 in the GPON system (e.g., 1310 nm burst-mode signals from ONUs 14 to OLT 12). These upstream signals are combined within splitter 22 using a multiple access protocol, usually time division multiple access (TDMA). For example, OLT 12 may control the transmission of the traffic from the individual ONUs 14 onto shared single fiber 18 via framing and synchronization (not shown) in order to provide time slot assignments for upstream communication.
As mentioned above, PONs do not use electrically-powered components to split the downstream signal. Instead, the signal is distributed among end users by means of passive optical splitters. Each splitter typically divides the signal from the transmission fiber 18 into N drop-line (or fan-out) fibers 24, where N is an integer and commonly depends on the manufacturer, the characteristics of drop-line fibers 24, the distance to the furthest ONU 14, and the like. PON configurations reduce the amount of fiber and service provider equipment needed compared with point-to-point architectures. In addition, a PON requires little maintenance and no electrical powering in the passive outside plant (the ODN), thereby reducing expense for network operators. However, the maximum transmission distance (referred to hereinafter at times as “reach”) between OLT 12 and the farthest away ONU 14, as well as the split ratio 1:N, are currently limited by various physical layer limitations and the PON protocol.
For example, although the GPON standard (ITU-T G.984) allows for a logical reach of 60 km and a maximum split ratio of 1:128, a 28 dB loss budget (i.e., acceptable power loss limit for a system) currently limits typical GPON deployments to a 20 km reach and a 1:32 split ratio. Of course, for a given loss budget, if a particular application needs only a relatively small split ratio (e.g., 1:16), then the reach may be longer (e.g., 24 km). Conversely, if an application utilizes only a relatively short reach (e.g., 10 km), then the split ratio may be larger (e.g., 1:64). However, some applications require both a long reach (e.g., 60 km) and a large split ratio (e.g., 1:64).
There have been several techniques attempted to extend the reach of GPON systems. In addition, GPON reach extenders have been standardized recently by the International Telecommunications Union (as explained fully in ITU-T G.984.6). While workable, the reach extension approaches considered in G.984.6 require the use of electrically-powered units in the outside plant—elements such as optical amplifiers or optical-to-electrical-to-optical (OEO) repeaters. As a result, these designs negate some of the advantages of purely passive systems and may not always be practical or cost effective for network service providers/operators, particularly in certain environments where no electrical power is available.
Future access networks will also require increased bit rates up to 10 Gbit/s in order to satisfy the ever-increasing traffic demands of system users. Indeed, a 10 Gbit/s PON (hereinafter referred to as “XGPON”) has recently been considered in ITU-T standard G.987.2. To ensure a smooth upgrade from GPON to XGPON for network operators, co-existence of both systems is considered mandatory, and will continue for some time to come. FIG. 2 illustrates an exemplary prior art combined GPON and XGPON system 30, where additional signal paths associated with the XGPON system are added in a straightforward manner to supplement the basic prior art configuration of FIG. 1. In comparing the two systems, GPON/XGPON system 30 is seen to include an additional source 32 and detector 34 disposed at OLT 12 to transmit and receive signals operating at the higher data rate (and using different transmit and receive wavelengths than those associated with GPON systems). In order to transmit these additional wavelengths along feeder fiber 18, a coarse wavelength division multiplexer (CWDM) 36.1 is included within OLT 12 to provide coupling between feeder fiber 18 and a pair of WDMs 36.2 and 36.3 specifically configured to operate at the GPON and XGPON system wavelengths, respectively. A “coarse” WDM simply refers to a WDM that operates with a wider spacing between the wavelengths being separated or combined and, as a result, may require less sophisticated and expensive components than a conventional WDM.
As before, ODN 16 of system 30 is a passive arrangement and includes a feeder fiber 18 and a remote node 20 (the remote node similarly including passive power splitter 22). Here, splitter 22 handles all four wavelengths; the pair of wavelengths associated with the upstream and downstream GPON signals and the pair of wavelengths associated with the upstream and downstream XGPON signals. In this prior art arrangement, splitter 22 is configured to simply split both downstream signals and transmit both along each fiber 24 (and, similarly, combine all of the upstream signals and couple into feeder fiber 18).
In the arrangement as shown in FIG. 2, each ONU 14 is configured to include elements to separate the GPON and XGPON signals such that only either the GPON signals or the XGPON signals are transmitted and received. For example, ONU 14.1 is shown as including a WDM 38.1 and a wavelength blocking filter (WBF) 40.1 that are used in conjunction with a transmitter 37.1 and a receiver 39.1 to communicate with OLT 12. For the sake of discussion, it is presumed that ONU 14.1 includes a transmitter and a receiver configured for the basic GPON system. As mentioned above, each fiber 24 supports the propagation of all signals. Therefore, the downstream input to WDM 38.1 will consist of both the GPON signal (the desired signal) and the XGPON signal (the undesired signal for ONU 14.1). In this prior art arrangement, therefore, WBF 40.1 is configured to “block” the XGPON signal from continuing to propagate and reach receiver 39.1.
Similarly, presuming that ONU 14.2 is associated with the XGPON communication system, WBF 40.2 is configured to block the GPON downstream wavelength, allowing only the XGPON downstream signal to reach receiver 39.2.
While this arrangement is capable of providing communication for both the GPON and XGPON systems through a single network, it requires each ONU to include a WDM and associated WBF in order to ensure that each ONU 14 receives signals from its associated system. The overall network itself remains limited in terms of both its reach and split ratio since it needs to accommodate all of the different wavelengths associated with each transmission system.
Thus, a need remains for an optical communication system that allows for an XGPON system to coexist with a GPON system that retains the truly passive nature of the distribution network while still providing opportunities for extended reach and/or increased splitting ratio.